Method for correcting telephone signal degradation caused by the length of the analog line via the receiver

ABSTRACT

The present invention relates to the field of telephony and specifically to a method for correcting the degradation of telephone signal quality caused by analogue lines. The invention describes managing, via the receiver, a specific correction of telephone signal degradation caused by the length of the analogue line between the parties. Said line length between the receiver and the telephone switchboard to which said receiver is connected is stored in the receiver. Advantageously, the line lengths between the receiver of the called party and the corresponding switchboard are stored, for example in the director of the apparatus. During the call, said line length is taken into account by a specific correction means for said correction inside the receiver.

The present invention concerns the field of telephony and moreparticularly a method of correcting degradations in the quality of thetelephone signal caused by the analogue lines.

Unlike digital telephones such as mobile telephones or IP (InternetProtocol) telephones, analogue telephones are connected to a telephoneswitchboard by an analogue line the length of which may vary from fewmetres to several kilometres. The telephone signal carrying the voice ofthe speaker is transmitted on this line in an analogue fashion. Thisanalogue transmission gives rise to a degradation in the telephonesignal. This degradation results in an attenuation and a change intimbre of the voice. It is dependent on the total length of the analogueline between the two speakers, whether on the caller side or on the calldestination side.

Today, in telephones of the prior art, this degradation is notspecifically corrected. Some telephones include an equaliser in thereceiver for generating a gain curve as a function of the frequency forthe signals recorded by the microphone or emitted by the loud speaker.This equaliser is used to correct the deformation of the telephonesignal due to the physical properties of the receiver in which themicrophone is inserted and the loudspeaker. Some receivers offer thepossibility for the user to adjust this equaliser according to theirtastes. This adjustment is then applied to all calls and does not takeaccount of the analogue line lengths between the speakers.

The invention aims to solve the above problems by the management by thereceiver of a specific correction to the degradation of the telephonesignal due to the analogue line length between the speakers. This linelength between the receiver and the telephone switchboard to which it isconnected is stored on the receiver. Advantageously, the line lengthbetween the receiver of the destination and its telephone switchboardare stored, for example, within the directory of the apparatus. During acall, this line length is taken into account by specific correctionmeans for this correction within the receiver.

The invention concerns a telephone receiver that comprises means ofstoring an analogue line length between the subscriber and thedestination of a call and means of specifically correcting thedegradation of the telephone signal caused by the analogue line length.

According to a particular embodiment of the invention, the means ofstoring the analogue line length comprise means of storing a localanalogue line length between the subscriber and the telephone networkand means of storing a distant analogue line length between thetelephone network and the destination of the call. The means ofspecifically correcting the degradation of the telephone signal causedby the distant analogue line length comprise means of correcting thelocal and distant analogue line length.

According to a particular embodiment of the invention, the receivercomprises means of selecting an operating mode among the followingmodes:

-   -   a disengaged correction mode where no correction to the        degradation of the telephone signal caused by the analogue line        length is made;    -   a local correction mode where only the correction to the        degradation of the telephone signal caused by the local analogue        line length is made;    -   a distant correction mode where only the correction to the        degradation of the telephone signal caused by the distant        analogue line length is made;    -   a total correction mode where the correction to the degradation        of the telephone signal caused by the local and distant analogue        line length is made.

According to a particular embodiment of the invention, the correctionmeans comprises a digital equaliser programmed by a continuous gainfunction according to the analogue line distance to be corrected and thefrequency of the telephone signal.

According to a particular embodiment of the invention, the correctionmeans comprises means of applying gain steps according to the analogueline distance to be corrected and the frequency of the telephone signal.

According to a particular embodiment of the invention, the means ofapplying the gain steps consist of analogue filters.

The invention also concerns a method of correcting the degradation ofthe telephone signal caused by the analogue line length between thespeakers that comprises a step of obtaining the analogue line length tobe corrected and a step of programming the correction means according tosaid analogue line length obtained.

The features of the invention mentioned above, as well as others, willemerge more clearly from a reading of the following description of anexample embodiment, said description being given in relation to theaccompanying drawings, among which:

FIG. 1 illustrates the global architecture of the telephone network,

FIG. 2 illustrates the connection of an analogue receiver to thetelephone network,

FIG. 3 illustrates the degradations affecting the telephone signaltransmitted by analogue line,

FIG. 4 illustrates the architecture of an example embodiment of theinvention,

FIG. 5 illustrates the correction steps used in a particular embodimentof the invention.

The modern telephone system consists today many networks for connectingtogether receivers functioning according to different technologies. Thisis illustrated schematically by FIG. 1. The conventional telephonenetwork, referred to as STN, standing for Switched Telephone Network,1.1, makes it possible to connect analogue receivers 1.5 and 1.8. Whenthe mobile telephone was developed, the STN network was connected tothis mobile telephone network 1.2, referred to as the GSM network(Global System for Mobile communications). Mobile handsets 1.4 and 1.7can thereby communicate with each other, but also with analoguereceivers 1.5 and 1.8. In parallel, the global data transfer network,the internet, 1.3 has developed, connecting together various informationprocessing devices. On this network, digital voice transfer technologieshave been developed, opening the way to the appearance of digitaltelephone on the IP network, known by the abbreviation VoIP (Vox overIP) which now enables digital receivers based on these technologies tobe connected. These receivers, integrating computers, make it possibleto establish telephone communications with each other and also toanalogue receivers in the STN network and mobile handsets in the GSMnetwork. The term GSM network covers here all mobile telephony networks,whatever the protocol actually used, such as GPRS, UMTS or protocolsgrouped together under the 3G protocol name.

Here telephone receiver means any electronic device for establishing atelephone call. This device includes in particular analogue telephoneterminals including radio handsets of the DECT (Digital EnhancedCordless Telephone) type, mobile telephony terminals, computers providedwith telephony software, some personal assistants, some GPS terminals onboard vehicles or the like.

Apart from analogue handsets connected to the STN network, the majorityof these handsets are digital. Analogue receiver means a receiveremitting the telephone signal transmitting the voice in an analoguefashion, typically over an analogue line. Falling within this categoryare wireless or cable telephones, typically according to the DECT orCat-IQ (Cordless Advanced Technology—internet and quality) standards,where the base is connected in an analogue fashion to the telephoneexchange although communicating digitally with the handsets strictlyspeaking according to a radio transmission technology. In the lattercase, the concept of telephone receiver is extended to the base, theinvention being able to be implemented within handsets or in the base.

Digital receiver means a receiver where the telephone signaltransmitting the voice is converted digitally on the receiver and wherethis receiver emits a digital telephone signal in order to transmit thevoice to the network.

FIG. 2 illustrates the functioning of the transmission of the telephonesignal between an analogue receiver and the telephone network. Thereceiver 2.1 is connected by an analogue line to the telephone exchange2.2. The exchange is itself connected by a digital communication networkto the telephony network 2.3. When the terminal 2.1 establishes acommunication with a speaker, his voice is transformed into an analoguetelephone signal by the microphone of the receiver. This telephonesignal is then transmitted over the analogue line to the telephoneexchange 2.2. This analogue telephone signal is then converted todigital by the exchange. It is then a digital telephone signaltransporting the voice that is transmitted via the telephone network 2.3to the destination.

When this destination is also equipped with an analogue receiver, thereverse operation is performed. The telephone signal arrives digitallyas far as the telephone exchange of the destination. It is thenconverted to analogue by this exchange and then transmitted in ananalogue fashion to the receiver via the analogue line connecting theexchange to the receiver in order to be reproduced by the loudspeaker ofthe receiver. The voice of the destination follows an identicaltransmission scheme in the reverse direction.

When this destination is equipped with a digital receiver, the digitaltelephone signal is transmitted to the receiver and is converted thereinto analogue within the receiver in order to supply the loudspeaker. Herealso, the voice of the destination follows an identical path in thereverse direction. In this case, the line length is zero on thedestination side.

FIG. 3 illustrates the degradation of the telephone signal as a functionof the frequency for different line lengths. It can be seen that thetelephone signal is attenuated overall. This attenuation varies byaround 6 dB in the 300 Hz frequency range between a zero distance and adistance of 3.5 km. At the latter distance, it is also seen that theattenuation is greater by 8 dB for a 1300 Hz frequency compared with afrequency of 300 Hz. During a communication involving at least oneanalogue receiver, the telephone signal of the voice transmitted istherefore degraded. This degradation is non-uniform according to thefrequency and depends on the total analogue line length over which thetelephone signal passes between the speakers.

It is possible to correct this degradation by applying a suitableequalisation to the telephone signal. This correction makes it possibleto correct the relatively greater attenuation at high frequenciescompared with low frequencies. It may for example be done by applyingthe following formula:

$C = {{L \cdot \frac{20}{7}}\left( {{{Log}_{10}(F)} - 2} \right)}$

where F is the frequency expressed in Hz, L is the length of the line tobe corrected in km and which gives the correction C for this frequencyin the form of a gain in dB to be applied to the telephone signal.

FIG. 4 illustrates an example embodiment of the invention. In thisexample, the invention is implemented in the receiver of a domesticwireless telephone system of the DECT type 4.1. In this system, thetelephone signal is received digitally 4.6 from the DECT base itselfconnected in an analogue fashion to the telephone exchange. Thetelephone signal is then decoded by the codec 4.4. It then undergoes anequalisation treatment carried out by the equaliser 4.5 before beingconverted into analogue by an analogue to digital converter or ADC inorder to be emitted by the loudspeaker 4.8. The voice captured by themicrophone 4.7 is converted to digital by an ADC before undergoingequalisation treatment by the equaliser 4.5 before being encoded by thecodec 4.4 in order to be sent to the network by the wireless digitalinterface 4.6. The receiver functions under the control of a processor4.2. This processor executes a set of control software housed in amemory space 4.3. In particular it controls the coding decoding module4.4 and the digital equaliser 4.5. This equaliser is typically used tocorrect deformations in the sound caused by the particularimplementation of the transducers within the receiver. According to theinvention, the memory 4.3 also constitutes a means of storing theanalogue line length between the subscriber and the telephone systemreferred to as the local analogue line length. Typically, this memoryalso contains the telephone directory of the subscriber. Advantageously,this directory also contains, for each destination present in thedirectory, information on the analogue line lengths between the receiverof the destination and the telephone network, referred to as the distantanalogue line length. This line length is set to zero in the case of amobile receiver or one based on IP.

The presence in the information storage means 4.3 of these analogue linelengths, both local and distant, enables the call management software toprogram the equaliser 4.5 in order to specifically correct thedegradation in the telephone signal caused by the analogue line length.This digital equaliser can be programmed by means of the functiondescribed in the above equation, which then constitutes a continuousgain function as a function of the analogue line length to be correctedand the frequency of the telephone signal. This software, with the linelength information present in the storage means of the memory 4.3 andthe equaliser 4.5, then constitutes a means of specific correction ofthe degradation in the telephone signal caused by the analogue linelength between two speakers during a call.

Advantageously, the same correction is applied to the telephone signalreceived from the destination of the call and to the telephone signalsent to the destination of the call.

This correction means then comprises means of calculating the totalanalogue line length associated with a given call, means of programmingthe equaliser for correcting the degradation in the telephone signalcaused by this analogue line length. It then implements a method ofcorrecting the degradation in the telephone signal caused by theanalogue line length that comprises a step of obtaining the analogueline length to be corrected and a step of programming the correctionmeans as a function of said analogue line length obtained.

Advantageously, the correction can be done according to several modes.In a first mode, referred to as total mode, the equaliser is programmedto correct the degradations caused by the whole of the analogue linelength, local and distant, between the subscribers. In a second mode,referred to as local mode, only the local analogue line length iscorrected and not the distant analogue line length. In a third mode,referred to as distant mode, only the distant analogue line length iscorrected and not the local analogue line length. Finally, in a fourthmode, referred to as disengage mode, no correction is made.Advantageously, these different modes can be selected by the user duringa call. This makes it possible among other things to be able to manage acall between two appliances having the invention available and avoidinga double correction that will be made by the two appliances and whichwould both correct the whole of the degradation both local and distant.In this embodiment, the correction is then made continuously accordingto the frequency.

The invention can be implemented by an analogue telephone receiver asdescribed in the example embodiment. However, it can also be implementedby a digital telephone receiver. In this case, the local analogue linelength is zero, and only the degradation of the telephone signal causedby the distant analogue line length can where necessary be corrected.

The correction can be entrusted to an equaliser present in the receiverfor other corrections or to a specific equaliser dedicated to thiscorrection according to circumstances.

Alternatively, the correction can be made not continuously but byapplying gain steps as a function of the frequency and distance. Thisembodiment may be advantageous on low-cost receivers. FIG. 5 illustratesan example of steps applicable in this embodiment. In this table, thelow frequencies are the frequencies between 300 Hz and 1 kHz, while thehigh frequencies are frequencies between I kHz and 3 kHz. It indicatesfor each distance the gain values to be programmed for correcting thedegradations in the telephone signal caused by the analogue line length.

Alternatively, for example in a completely analogue receiver where thetelephone signal is not converted to digital, the correction may be madeby applying analogue filters. Advantageously, the method then usessteps, for example those in FIG. 5, to select the correct filtersaccording to the distance and the frequency of the telephone signal.

1. Telephone receiver intended to be connected to a telephone networkvia a local analogue line and comprising correction means for correctingthe degradation of a telephone signal caused by the length of the localanalogue line, characterised in that it comprises: means (4.3) forstoring information on the length of the local analogue line between thetelephone receiver claimed and the telephone network; means (4.3) forstoring information on the length of a distant analogue line connectinga distant telephone receiver and the telephone network; and in that thecorrection means are adapted also to correct, according to the lengthinformation stored, the degradation of the telephone signal caused bythe length of the distant analogue line, the said correction means beingused during a call between the telephone receiver claimed and thedistant telephone receiver.
 2. Telephone receiver according to claim 1,characterised in that it comprises means for selecting an operating modefrom the following modes: a first correction mode in which no correctionof the degradation of the telephone signal caused by the length of thelocal and distant analogue lines is made; a second correction mode inwhich only a correction of the degradation of the telephone signalcaused by the length of the local analogue line is made: a thirdcorrection mode in which only a correction of the degradation of thetelephone signal caused by the length of the distant analogue line ismade; a fourth correction mode in which a correction of the degradationof the telephone signal caused by the length of the local and distantanalogue lines is made.
 3. Telephone receiver according to claim 1,characterised in that the correction means comprise a digital equaliserprogrammed by a continuous gain function according to the distance ofthe analogue line to be corrected and the frequency of the telephonesignal.
 4. Telephone receiver according to claim 1, characterised inthat the correction means comprise means for applying gain stepsaccording to the information on the analogue line length to be correctedand the frequency of the telephone signal.
 5. Telephone receiveraccording to claim 4, characterised in that the means for applying thegain steps consist of analogue filters.
 6. Method of correcting thedegradation of the telephone signal caused by the analogue line lengthbetween the first and second telephone receivers, the said methodcomprising a step of correcting the degradation of the telephone signalcaused by the length of a local analogue line connecting the firsttelephone receiver to a telephone network, the said method being used bythe said first telephone receiver and characterised in that itcomprises: a step of obtaining information on the length of the localanalogue line, a step of obtaining information on the length of adistant analogue line connecting the second receiver to the saidtelephone network; and a step of correcting the degradation of thetelephone signal according to the length information obtained.